Cermet and method of producing it

ABSTRACT

In order to improve the toughness characteristics of a cermet alloy, while retaining high resistance to wear, a composition is disclosed which contains 30 to 60% by weight of Ti, 5 to 20% by weight of W, 5 to 15% by weight of Ta, in which up to 70% of the Ta can be replaced by Nb, and 5 to 25% by weight of Ni and/or Co binder with more than 80 mole %, relative to the above transition elements of carbon and nitrogen. The composition is prepared by grinding, compressing and sintering a solid, powder-form mixture containing (Ti,W,Ta,Nb)C powder, Ti(C,N) powder, and WC powder, each powder having a particle size &lt;1.5 μm, plus Ni powder and/or Co powder. The mixture includes the following ingredients: (a) (Ti,W,Ta,Nb)C with a mean particle size &lt;1.5 μm, this mixed carbide containing 20 to 50% by weight of TiC, 20 to 40% by weight of WC, and 20 to 40% by weight of (Ta, Nb)C; (b) Ti(C,N), with a mean particle size &lt;1.5 μm and an N/(C+N) ratio &lt;0.7; WC with a mean particle size &lt;1.5 μm; and (d) nickel and/or cobalt.

FIELD OF THE INVENTION

The invention relates to a cermet with a hard material phase of 95 to75% by mass, the balance being binders of cobalt and nickel. Theinvention relates further to method of producing such a cermet bygrinding a carbide, nitride and/or carbonitride of titanium and tungstenand optionally further elements of the Groups IVa to VIa of theclassification of elements, as well as a binder metal such as a cobaltand/or nickel containing mixture which is subsequently pressed andsintered.

BACKGROUND OF THE INVENTION

The JP-A-60002646 describes an alloy with a ternary hard material phaseof WC, (Ti,W,Ta/Nb)C and (Ti,W,Ta/Nb)CN, whereby the alloy contains 5 to15% by weight Co, 5 to 5% by weight (Ti,W)C, 50% by weight (Ti,W)CN, 1to 15% by weight TaC, NbC and/or (Ta,Nb)C, the balance WC andimpurities.

Cermets consisting of 25 to 50% by weight titanium nitride,

10 to 30% by weight titanium carbide, 5 to 25% by weight tantalumcarbide, niobium carbide and/or zirconium carbide, 10 to 25% by weighttungsten carbide and 7.5 to 25% by weight binders of cobalt and/ornickel, as well as optionally 0.01 to 1% by weight aluminum added to thebinder and also a process for the production of the cermet are knownfrom DE 34 18 403 C2. In the mentioned process first a formed body isproduced by preliminary compression from the powder mixture of the abovecomposition and this formed body is sintered at over 1400° C. in anitrogen atmosphere, at a pressure between 0.13 and 133 mbar.

In order to improve the toughness, U.S. Pat. No. 4,778,521 proposes acermet composition consisting of 20 to 92% by weight titaniumcarbonitride, 5 to 50% by weight tungsten carbide and 3 to 30% by weightof a binder and has a triphase particle size microstructure comprising acore phase which is rich in titanium carbonitride, an intermediate phaserich in tungsten carbide surrounding the so-called core phase and anouter phase of titanium/tungsten carbonitride surrounding the previouslymentioned intermediate phase. In order to produce this cermet, titaniumcarbide powder, tungsten carbide and nickel are mixed and furthertreated as shown above.

For improving the mechanical characteristics, and especially forimproving the resistance against plastic deformation, the EP 0 270 509B₁ proposes to heat a mixtures of powders of TiC and (Ta, Nb) C and/orTaC after they have been mixed, and to comminute into a powder the solidsolution resulting after cooling. This powder should then be mixed withcarbides and nitrides or carbonitrides of metals of the Groups IVa toVIa of the classification of elements, as well as with a binder, pressedand sintered. The thereby produced body has a carbonitride component asa biphase mixture with a phase poor in nitrogen but rich in titanium andtantalum and a second phase which is rich in metal components of GroupVI and in nitrogen. The first phase forms the core and is surrounded bythe second phase as a marginal zone, which at the same time forms themain border surface with the binder metal alloy.

Especially for chip-removal machining operations with high cuttingspeeds it is proposed to prepare at first a carbonitride mixture of thecomposition (Ti, Ta, W)(C, N) and to mix with a binder the carbonitridepowder mixture obtained this way, to grind it and to compress it into agreen compact and subsequently to sinter it under nitrogen.

However according to EP 0 302 635 A1 there are two phases, the first ofwhich (the core phase) has a carbonitride which is poor in titanium andnitrogen, while the second phase surrounding the core consists ofcarbonitrides rich in titanium and nitrogen. This is supposed to beachieved for instance by mixing, compacting and sintering titaniumnitride or titanium carbonitride with a carbonitride of tungsten ortitanium and optionally with one or further elements of Groups IVa toVIa of the classification of elements.

According to EP 0 417 333 A1 for the production of a cermet alloy in thebeginning a first powder for the formation of a core structure ofcarbides and carbonitrides containing titanium and a second powder forthe formation of the marginal zone of titanium nitride, tantalum carbideand tungsten carbide, as well as a third powder of a binder metal, suchas cobalt and nickel are prepared, and the entire mixture thereof iscompacted and sintered.

The EP 0 406 201 A1 describes a carbonitride alloy with at least 80% byvolume hard materials which form duplex structures of core zones andmarginal zones and which constitute 10 to 80% by volume of the entirehard material phase. The carbonitride metals are selected from theGroups IVa and VIa of the classification of elements.

The DE 40 00 937 A1 describes a process for the production of cermetstarting out from powdery titanium carbide to which, besides the binder,mixed carbides of the elements tantalum, niobium, tungsten, or tantalumand tungsten, or titanium, tantalum and tungsten are admixed. Thetitanium carbide can also be partially enriched with zirconium as astarting material.

OBJECT OF THE INVENTION

It is the object of the present invention to indicate a cermet compoundas well as a process for its production, wherein the toughness as wellas the wear resistance during machining are considerably improved whencompared to the state of the art cermets.

SUMMARY OF THE INVENTION

The object of the invention is achieved by a cermet with a hard materialphase containing (Ti,W,Ta/Nb)C, (Ti,W,Ta,/Nb)CN, the balance a binderphase with a proprtion of

<5% by mass of Co and/or Ni, said cermet having a gross composition of30 to 60% by mass Ti, 5 to 20% by W, 5 to 11% by mass Ta which can bereplaced up to 70% by Nb, 5 to 25% by mass Ni and/or Co, with more than80% mole relative to the abovementioned Ti, W, Ta, and/or Nb of carbonand nitrogen, and which has at least 40% of hard material particles inthe stucture with a core-margin stucture with cores of Ti (C,N) N<C andhomogenneous marginal zones of (Ti, W, Ta/Nb)C, whereby the proportionof the hard material phase amounts to 95 to 75% by mass.

As far as no unitary core-margin structure exists over the entire cermetbody, 40 to 60% by volume of the hard material phase consists ofparticles with a core-margin structure with a core rich in nitrogen madeof TiCN (N/(N+C)≦0.7)) and a marginal zone made of (Ti, W, Ta and/orNb)C with Ti=50 to 65% by mass,

W=15 to 30% by mass, Ta and/or Nb=8 to 20% by mass and 60 to 40% byvolume of the hard material phase consisting of particles with ahomogeneous composition corresponding to the above-described marginalzone, whereby the proportion of binder phase ranges between 5 and 25% bymass.

Surprisingly it has been found that by following the afore-mentionedcomposition according to the invention, respectively the selection ofthe starting materials a much more fine-grained cermet alloy withimproved toughness characteristics can be created. The structureformation is uniformly fine-grained and has essentially a cubic NaClcrystalline structure (B₁ structure) with an evenly distributed bindermetal phase.

On account of the particles with a homogeneous composition, up to 5% byvolume of the hard material particles have a core composition of (Ti, W,Ta and/or Nb)C with 43 to 53% by mass titanium, 35 to 50% by masstungsten, 4 to 8% by mass tantalum and/or niobium, whereby the marginalzone corresponds with the above-mentioned composition. This means thatthe border surfaces of all hard material particles have the sameaforedescribed composition with respect to the binder phase.

According to a further embodiment the hard material phase hasexclusively a cubic B₁ -crystalline structure and has an average corediameter of <1.5μ in conditions of even distribution of the binder metalphase.

Further the cermet alloy can contain additionally (in % by mass) 0 to 12molybdenum, 0 to 5 vanadium, 0 to 5 chromium and/or 0 to 2 aluminum.

According to a further embodiment of the invention the binder is presentin a proportion of Ni/(Ni+Co)=0.2 to 0.8. Furthermore the nitrogen ispreferably selected in a proportion ranging between 0.2 and 0.8 inrelation to the sum of carbon and nitrogen.

Further it is preferred that in the binder metal phase considerablylarger amounts of tungsten than of titanium be dissolved, whereby thetungsten can be partially replaced by molybdenum, vanadium, chromium oraluminum.

The object of the invention relating to the process is achieved due tothe steps taken in accordance with the present process whose noveltyconsists in the fact that the starting powder mixture has the followingcomponents:

(a) 15 to 45% by mass (Ti, W, Ta,)C with an average particle size of<1.5μ, whereby this carbide mixture contains 20 to 50% by mass TiC, 20to 40% by mass WC and 20 to 40% by mass TaC, whereby up to 70% by massof Ta can be replaced by Nb.

(b) 3 to 15% by mass WC with an average particle size <1.5μ,

(c) 5 to 25% by mass of nickel and/or cobalt and

(d) the balance Ti(C, N) with an average particle size of

<1.5μ and N/(C+N)<0.7, preferably 25 to 65% by mass Ti(C,N).

This starting powder mixture, whose components a) and d) are prepared asmaster alloys, is ground, pressed into a green compact and sinteredaccording to the known state of the art.

According to a further embodiment of the process according to theinvention TiAl₂ C, molybdenum and/or molybdenum carbide, chromium and/orchromium carbide and/or vanadium carbide can be included in the startingmixture up to 5% by mass.

Preferably the mixed carbide (W, Ti, Ta, Nb)C is selected with a grainsize of 0.5μ and with the proportion of carbides WC:TiC:(Ta, Nb)C=1:1:1.Further the titanium carbide can be used in the starting substance withan average particle size of 1.2μ and in a proportion of N/(C+N)=0.75.

In a concrete embodiment example of the invention 29.6% by mass (W, Ti,Ta, Nb)C of an average grain size of approximately 0.5μ with equal partsof TiC, WC and (Ta,Nb)C (with Ta/(Ta+Nb)=0.9) and 43.4% Ti(C,N) with agrain size of approximately 1.2μ and a N/(N+C)=0.75 proportion of 9.3%by mass WC, 8.2% by mass Ni, 8.5% by mass Co and 1.0% by mass TiAl₂ Care mixed together, wet ground, compressed into a green compact andsubsequently sintered or pressure-sintered to a cutting tool. Thesintered material produced this way distinguishes itself over the cermetsintered materials known to the state of the art with a comparable grosscomposition by being considerably more fine-grained.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1a, b each show a scanning electron microscope structure photo of acermet, made of Ti(C,N), (Ti, W) (Ti,W,Ta; Nb)C, Ni and Co powder(enlargement of 3000:1 (FIG. 1b) and 5000:1 (FIG. 1a)). alloy A,

FIG. 2a, b show a scanning electron microscope structure photo of amaterial, made of TiN, TiC, WC, (Ta,Nb)C, Ni and Co powder (enlargement3000:1 (FIG. 2b) and 5000:1 (FIG. 2a)), alloy B,

FIG. 3a, b show a scanning electron microscope structure photo of thecompositions of the invention in the previously described embodiment inan enlargement of 3000:1 (FIG. 3b) and 5000:1 (FIG. 3a), alloy C.

Any comparison of FIG. 1a, respectively 2a with 3a or of 1b,respectively 2b with 3b shows that the cermet alloy of the invention isconsiderably more fine-grained than the alloys known to the state of theart and contains exclusively particles with a dark core, respectivelyparticles with homogeneous composition. In the shown scanning electronmicroscope photos backscattered electrons are used for producing theimage, i.e. dark structure components have a low density and aretherefore rich in Ti, light gray structure components have a higherdensity and are therefore rich in W and Ta. The binder phase appearsalmost white.

The cermet alloy C of the invention is used as material for a cuttinginsert of the geometry SPGN 120308 with a cutting edge chamfer between30 to 50μ and was compared with indexable inserts of the same geometrymade of alloys A and B. The results can be seen from the following data

1. Slotted shaft

Workpiece material: CK45N, Strength: 700 N/mm²

    ______________________________________                                        Cutting conditions:   v.sub.c = 250 m/min                                                           a.sub.p = 1.5 mm                                                              f = 0.2 mm                                                                    Number of impacts                                       Comparison cermet (alloy A)                                                                         146 (average value                                                            from 3                                                  Comparison cermet (alloy B)                                                                         210 cutting corners)                                    Cermet of the invention (alloy C)                                                                   275                                                     ______________________________________                                    

2. Bolt turning test with advance increase

Workpiece material: CK45N, Strength: 720 N/mm²

    ______________________________________                                        Cutting conditions:                                                                            v.sub.c = 250 m/min                                                           a.sub.p = 2.0 mm                                             Advance range    0.10-0.12-0.16-0.20-0.25-                                                     0.31 per advance step, 3 overruns                                             Number of overruns                                           Comparison cermet (alloy A)                                                                    7.8 (average value from 3                                    Comparison cermet (alloy B)                                                                    7.7 cutting corners)                                         Cermet of the invention (alloy C)                                                              9.5                                                          ______________________________________                                    

What is claimed is:
 1. Cermet with a hard material phase containing(Ti,W,Ta/Nb)C and (Ti,W,Ta/Nb)CN, the balance a binder phase with aproportion of>5% by mass of Co and/or Ni, characterized by a grosscomposition of 30 to 60% by mass Ti, 5 to 20% by mass W, 5 to 11% bymass Ta which can be replaced up to 70% by Nb, 5 to 25% by mass Niand/or Co, with more than 80% mole--relative to the abovementioned Ti,W, Ta, and/or Nb, of carbon and nitrogen, which has been prepared from asolid powdery starting mixture with 15 to 45% by mass(Ti,W,Ta)C and/or(Ti,W,Ta,Nb)C, 3 to 15% by mass WC, 5 to 25% by mass Co and/or Ni, thebalance Ti(C,N), each with a particle size <1.5μ through grinding,pressing and sintering and which has at least 40% of hard materialparticles in the structure with a core-margin structure with cores ofTi(C,N), N>C and homogeneous marginal zones of (Ti,W,Ta/Nb)C, wherebythe proportion of the hard material phase amounts to 95 to 75% by mass.2. Cermet according to claim 1, characterized in that the Ti(C,N) iscontained in the starting mixture in 25 to 65% by mass.
 3. Cermetaccording to claim 1, characterized in that 40 to 60% by volume of thehard material phase consists of particles with core-margin stucture witha nitrogen-rich core of TiCN with N/(N+C)≦0.7 and a marginal zone of(Ti,W,Ta)C with Ti=50 to 65% by mass, W=15 to 30% by mass, Ta=8 to 20%by mass and that 60 to 40% by volume of the hard material phase consistsof particles with a homogeneous composition corresponding to thepreviously described marginal zone, whereby in the hard material phasesup to 70% of the Ta can be replaced by Nb.
 4. Cermet according to claim3, characterized in that on account of the particles with homogeneouscomposition in addition up to 5% by volume of the hard materialparticles have a core composition of (Ti,W,Ta,Nb)C with 43 to 53% bymass titanium, 35 to 50% by mass tungsten, 4 to 8% by mass tantalumand/or niobium.
 5. Cermet according to claim 1, characterized in thatthe hard material phase has exclusively a cubic B₁ -crystallinestructure and an average core diameter <1.5μ with an even distributionof the binder metal phase.
 6. Cermet according to claim 1, characterizedin that additionally 0 to 12 Mo, 0 to 5 V, 0 to 2 Cr and/or 0 to 2 Alare contained in % by mass.
 7. Cermet according to claim 1,characterized in that the binder is present in a proportion ofNi/(Ni+Co)=0.2 to 0.8.
 8. Cermet according to claim 1, characterized inthat in the binder metal phase there are larger amount of tungsten thanof titanium.
 9. Cermet according to claim 6, characterized in that theamount of dissolved molybdenum, vanadium, chrome and tungsten is largerthan the amount of titanium.
 10. Cermet according to claim 1,characterized in that the nitrogen proportion in relation to the sum ofcarbon and nitrogen ranges between 0.2 and 0.8.
 11. A process forproducing a cermet with a hard material phase containing (Ti,W,Ta/Nb)C,(Ti,W,Ta/Nb)CN, the balance a binder phase with a proportion of>5% bymass of Co and/or Ni, said cermet having a gross composition of 30 to60% by mass Ti, 5 to 20% by mass W, 5 to 11% by mass Ta which can bereplaced up to 70% by Nb, 5 to 25% by mass Ni and/or Co, with more than80% mole relative to the abovementioned Ti, W, Ta, and/or Nb of carbonand nitrogen, and which has at least 40% of hard material particles inthe structure with a core-margin structure with cores of Ti(C,N) N>C andhomogeneous marginal zones of (Ti,W,Ta/Nb)C, whereby the proportion ofthe hard material phase amounts to 95 to 75% by mass, which comprisesthe steps of: (1) grinding a starting mixture which contains thefollowing components:(a) 15 to 45% by mass (Ti,W,Ta)C with an averageparticle size of <1.5 μm, whereby this carbide mixture contains 20 to50% by mass TiC, 20 to 40% by mass WC, and 20 to 40% by mass TaC,whereby the Ta can be replaced up to 70% by mass by Nb; (b) 3 to 15% bymass WC with an average particle size of <1.5 μm; (c) 5 to 25% by massnickel and/or cobalt; and (d) the balance Ti(C,N) with an averageparticle size of <1.5 μm and a proportion of N/(C+N)>0.7, to obtain aground mixture; (2) pressing the ground mixture obtained during step (a)into a green compact; and (3) sintering the green compact to obtain thecermet.
 12. Process according to claim 11, characterized in that thestarting powder mixture contains 25 to 65% by mass of Ti(C,N). 13.Process according to claim 11, characterized in that the starting powdermixture contains additionally up to 5% by mass of at least one of thecomponents TiAl₂ C, Mo, Mo₂ C, Cr, Cr₂ C₃ and V.
 14. Process accordingto claim 11, characterized in that the carbide mixture (W,Ti,Ta)C or(W,Ti, Ta,Nb)C is present in an average fine-graininess of 0.5μ in theproportion of the carbides WC:TiC:TaC: or (Ta,Nb)C=1:1:1.
 15. Processaccording to claim 11, characterized in that the Ti(C,N) is present inan average particle size of 1.2μ with a proportion of N/(C+N)=0.75.